Gibbons are the nearest relatives to humans that mate for life. They form extremely strong pair bonds and exhibit low sexual dimorphism, which means that males and females of the species are of roughly equal size, a testament to the fact that both sexes are on relatively equal footing. The coupled male and female will spend time grooming each other and (literally) hanging out together in the trees. But more recent research has found that these unions are not quite as uncomplicated as once thought. With mates occasionally philandering, and even sometimes dumping a mate, the gibbon mating culture has started to look perhaps a little bit more like ours.